Well, I just got this from another answer.

var StellarSdk = require('stellar-sdk')
var server = new StellarSdk.Server('https://horizon-testnet.stellar.org');

// Get a message any time a payment occurs. Cursor is set to "now" to be notified
// of payments happening starting from when this script runs (as opposed to from
// the beginning of time).
var es = server.payments()
  .cursor('now')
  .stream({
    onmessage: function (message) {
      console.log(message);
    }
  })

Well, I just got this from another answer.

var StellarSdk = require('stellar-sdk')
var server = new StellarSdk.Server('https://horizon-testnet.stellar.org');

// Get a message any time a payment occurs. Cursor is set to "now" to be notified
// of payments happening starting from when this script runs (as opposed to from
// the beginning of time).
var es = server.payments()
  .cursor('now')
  .stream({
    onmessage: function (message) {
      console.log(message);
    }
  })

To watch for a particular account you may try

var es = server.transactions()
    .forAccount(accountAddress)
    .cursor(lastCursor)
    .stream({
        onmessage: txHandler
    })

as explained here